Virtually all SNRs emit in radio waves, where the emission is entirely nonthermal in origin, resulting from synchrotron radiation from relativistic electrons spiraling around magnetic fields in the shock. Synchrotron emission is characterized by a featureless power-law spectrum, where the radio flux, \(S_{\nu }\)  is given by \(S_{\nu } \propto \nu ^{-\alpha }\) , where \(\nu \)  is the frequency and \(\alpha \)  is the spectral index, which depends on the energy distribution of the electron population.
For more detail, see Chapter “acceleration".
In Figure REF , we show an example of a remnant, G1.9\(+\) 0.3, which is still in the free expansion phase [1]}. The acceleration of electrons up to GeV energies is sufficient to produce radio synchrotron emission.
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